A new reversible molecular phase transition material [HN(C4H9)3](fumrate)0.5·(fumaric acid)0.5 (1) has been successfully synthesized. Differential scanning calorimetry measurement shows a pair of reversible peaks at 181.9 and 178 K on heating and cooling modes, respectively. The large thermal hysteresis of ∼3.9 K discloses its reversible first-order structural phase transition. Specific heat capacity and dielectric constant measurements around Tc further confirm its phase transition behaviors. The detailed structural analyses of 1 at variable temperatures reveal that its structural phase transition is mainly accomplished by the order–disorder transformations of carboxyl oxygen atoms and distinct reorientations among cations and anion-acid infinite sheets together with proton-dynamic motion. All these results open a new way to construct potential phase transition materials through the selection of flexible aliphatic cations with simple carboxyl-based anions.